Heretofore, it is well known that an electric power steering apparatus is one of electrical control systems that are supplied from a battery as a power supply. The electric power steering apparatus drives a motor based on a steering torque inputted by a driver and so on to provide a steering system with an assist force. Usually, since an electricity consumption of such an electric power steering apparatus is considerably high, there is a possibility that the influence of battery degradation is large and it is impossible to obtain a desired assist force when the voltage drops. Therefore, in view of reliability and safety of the electric power steering apparatus, it is necessary to diagnose a state of the battery that is supplied to plural electrical control systems at a beginning time or a shutdown time of driving of the electric power steering apparatus.
An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and the motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of such the electric power steering apparatus will be described with reference to FIG. 1. A column shaft 2 connected to a steering wheel (handle) 1 is connected to tie rods 6 of steered wheels through reduction gears 3, universal joints 4A and 4B, and a rack and pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1 in accordance with torsion of a torsion bar, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 30 for controlling the electric power steering apparatus from a battery 14, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 30 calculates a current command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a velocity Vel detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on the calculated current command value I.
The control unit 30 mainly comprises a CPU (or an MPU (Micro Processor Unit) or an MCU (Micro Controller Unit)), and general functions performed by a program within the CPU are shown in FIG. 2 with respect to a vector control method. The vector control is a method that independently sets a q-axis for controlling a torque which is a coordinate system of the rotor magnet and a d-axis for controlling the strength of a magnetic field, and controls currents of the q-axis and the d-axis that have a relation of 90-degree. In general, a three-phase brushless DC motor is used as the motor 20 in the vector control.
The control unit 30 comprises a current command value calculating section 31. The current command value calculating section 31 inputs the steering torque T from the torque sensor 10 and the velocity Vel from the velocity sensor 12, simultaneously inputs a motor angle θ and an angular speed ω that are obtained by converting an output of a resolver 201 provided at the brushless DC motor 20 as a rotation angle sensor by a resolver-to-digital converting circuit (RDC) 202, and calculates a d-axis current command value Idref and a q-axis current command value Iqref by referring to an assist map.
The calculated d-axis current command value Idref and the q-axis current command value Iqref are inputted to a two-phase/three-phase converting section 32, and converted to current command values of three phases Iaref, Ibref and Icref in accordance with the motor angle θ. The converted current command values Iaref, Ibref and Icref are inputted to subtracting sections 33-1, 33-2 and 33-3 respectively, a deviation ΔIa between the current command value Iaref and the motor current Ia obtained by a current detector 37-1, a deviation ΔIb between the current command value Ibref and the motor current Ib obtained by a current detector 37-2, and a deviation ΔIc between the current command value Icref and the motor current Ic obtained by a subtracting section 37-3 are calculated, respectively. These deviations ΔIa, ΔIb and ΔIc are inputted to a current control section 34 such as PI (proportional-integral control), and controlled voltage command values Varef, Vbref and Vcref are outputted. Then, the voltage command values Varef, Vbref and Vcref are inputted to a PWM control section 35. The PWM control section 35 performs a PWM control based on the voltage command values Varef, Vbref and Vcref, and inputs PWM-controlled PWM signals to an inverter circuit 36. The inverter circuit 36 is supplied from the battery 14 as the power supply through a power relay 13, supplies the currents Ia, Ib and Ic to the motor 20 based on the PWM control signals from the PWM control section 35, and performs the vector control with respect to the motor 20 so as to set the deviation ΔIa obtained by the subtracting section 33-1, the deviation ΔIb obtained by the subtracting section 33-2, and the deviation ΔIc obtained by the subtracting section 33-3 as zero, respectively.
With respect to the battery 14 that is supplied to the electrical control systems of such the electric power steering apparatus, in order to normally and stably assist the steering operations of the driver, it is necessary to maintain the power voltage of the battery in a given stable range (for example, 10V-15V). However, failures such as battery degradation (voltage drop) due to various causes, occur. Therefore, before the battery 14 degrades to a degree becoming a hindrance to normal driving of the vehicle, it is necessary to detect the degradation of the battery to inform the driver and prompt charging or exchanging.
In order to solve such a problem, in Patent Document 1 (Japanese Patent No. 4270196 B2), a battery state diagnosis apparatus that is capable of diagnosing a battery status by energizing an electric motor without varying a vehicle status even in a vehicle state control apparatus without plural electric motors by limiting only a d-axis armature current in a dq-axes coordinate system which comprises of a d-axis being an action axis of a magnetic flux created by a permanent magnet of a rotor of a brushless DC motor and a q-axis that is perpendicular to the d-axis to less than or equal to a given upper-limit current value and passing the limited d-axis armature current, and not passing a q-axis armature current, is proposed.
Further, in Patent Document 2 (Japanese Patent No. 4243146 B2), a battery state determining apparatus in an electric power steering apparatus with plural actuators that drives at least one of the plural actuators to steer in a right direction and simultaneously drives other at least one of the plural actuators to steer in a left direction, and controls output torques of actuators that are driven to steer in the left and right directions respectively so that the wheels are not steered, and determines the battery state based on the amount of descent of a terminal voltage outputted from a voltage sensor during driving the above actuators, is proposed.